1. Field of the Invention
This invention relates generally to automated air-monitoring equipment used to analyze and determine the concentration of compounds, such as volatile organic compounds, in sampled air. More specifically, the invention relates to an apparatus for sampling air for two or more similar or dissimilar compounds simultaneously at one or more locations, that allows for different sample volume requirements and/or flow restrictions for each sample line.
2. Description of Related Art
Automated air-monitoring equipment typically collects a known volume of air that is analyzed for compounds of interest, including volatile organic compounds and chemical-warfare agents, to determine the concentration (e.g., unit mass per unit volume) of the compound in the sampled air. The air monitor or sampling device may perform direct or indirect concentration measurements. Direct methods include measuring the concentration of the target compound in a static sample of air or dynamically measuring the concentration in a flowing air stream. An indirect method typically involves collecting the compound of interest from an air sample by some means before analysis; this approach is used to pre-concentrate the analyte for low-level detection. For direct and indirect monitoring cases, the volume of the sampled air must be known to determine the concentration of the target compound. For indirect methods, the concentration of the analyte in the sampled air can be back-calculated based on the quantitative analysis of the collected compound and the volume of sampled air.
The air monitor may use a vacuum pump or other means to sample a known volume of air that is subsequently analyzed for the compound of interest. The usual methods to measure the volume of the air sample include collecting the air sample in a fixed, known volume container, or by measuring the sample flow rate and the sampling time, the product of which yields the sample volume.
Air monitors usually perform xe2x80x9cpoint samplingxe2x80x9d, which means that the air monitor collects an air sample at one particular location. To sample more than one point automatically, the monitor must be designed to collect air samples from multiple locations either simultaneously or sequentially. When xe2x80x9cmultiple-point samplingxe2x80x9d is required, two or more sample lines (i.e., flexible or rigid tubing) are used, and manual or automated devices incorporating pneumatically or electrically controlled valves can be used with vacuum sampling to switch the sampling between the different lines leading to different sampling points. These switching sampling devices, sometimes called xe2x80x9cstream-selectionxe2x80x9d devices or systems, simply alternate sampling between different points.
Automated multiple-point, stream-selection sampling devices that are based on vacuum sampling generally collect equal volumes of air from each of the multiple points by using a fixed sample flow rate and a fixed sampling time at each point. That is, the volume of gas collected is the same for each sampling point because the valve(s) merely switch between lines at predetermined fixed intervals.
In these stream-selection systems, the volume of the air sample for each sampling point equals the volume of air collected while sampling that point. The two sample lines converge at the inlet of the sampling device. The expected concentration of the compound may be different at the different sampling points, the required monitoring level (i.e., target concentration level) may differ for each of the multiple sampling points, or the volumes of air required for the analyses may vary depending on the specific compounds being sampled. Additionally, in some instances dissimilar compounds must be sampled through separate sample lines at the same or different locations. There is no simple, reliable way to handle these differences and variations by controlling the portion of the total volume of air sampled from each sample line.
In theory, variable restrictions could be added to one or more of the sample lines to tune the proportion of sample flow in each line to the desired ratio. However, this approach is impractical because variable restrictions are difficult to work with and are generally unreliable for calibrating to a particular flow rate. Variable restrictors can also impede the collection efficiency of the analyte or may cause problems of chemical xe2x80x9ccarryoverxe2x80x9d or xe2x80x9cmemory,xe2x80x9d resulting from the retention of the analyte by the restriction.
As a specific example where different sampling requirements exist, sampling of air for the concentration of various chemical-warfare agents requires air sample volumes that are different for certain of the agents. Three such chemical-warfare agents are: (1) HD, a designation used for bis-(2-chloroethyl) sulfide, a blister agent commonly known as mustard gas; (2) GB, a designation used for isopropyl methylphosphonofluoridate, a nerve agent commonly known as sarin; and (3) VX, a designation used for O-ethyl S-[2-(diisopropylamino) ethyl] methylphosphonothiolate, another nerve agent. Monitoring for the agents HD, GB and VX typically requires sampling of different volumes of air for each agent. To determine the concentration of each agent, an air sample is pulled through the inlet of a chemical monitor such as the MIMCAMS, which is a miniature gas-chromatographic system manufactured and sold by CMS Field Products (xe2x80x9cCMSxe2x80x9d) of O.I. Analytical. MINICAMS can be configured with one of several different gas-chromatographic detectors, such as a flame-ionization detector, a photoionization detector, a flame-photometric detector, or another detector that is sensitive to the volatile organic compounds of interest.
During the sampling portion of the MINICAMS monitoring cycle, air is pulled into the inlet of the MINICAMS through a preconcentrator tube containing a sorbent which traps the compounds of interest. The sample line is typically xc2xc-inch Teflon tubing having a length of 100 feet or more. Knowing the total volume of the air accurately is critical because the compound concentration must be based on the measurement of the amount of the compound trapped on the sorbent in relation to the total air sample volume.
Further complicating the monitoring of these agents, HD or GB propagate down the sample line and can be directly sampled, but agent VX must be chemically converted to its G-analog at the remote sample point before the G-analog propagates down the sample line and its concentration is determined. This conversion is necessary because VX has a very low vapor pressure and is difficult to transport through the sampling line. Converting agent VX to the G-analog, which is similar to nerve agents such as GB, is achieved by pulling the sampled air through a V-to-G conversion filter, or pad, impregnated with silver fluoride (AgF), which reacts with any VX present in the air sample to form the G-analog. The filter or pad is at the remote (distal) end of the sample line. The concentration of VX is proportional to the concentration of the G-analog. Although agent GB will pass through the conversion filter or pad, agent HD will not. Therefore, it is not possible to simultaneously sample agent HD and agent VX through the same sample line.
This invention solves the above problems and disadvantages by providing an apparatus for controllably varying the sample volume for each of two or more sample lines used with air monitoring equipment. The invention can sample different volumes of air from different sampling points by varying the sampling time at each point. The sample volume is varied depending on the compound or compounds of interest sampled through each line. To vary the volumes, time values are set or programmed by the operator to switch valves based on desired monitoring levels for different target analytes and the relative sensitivities of the monitoring system to detect the target analytes. The invention also can be used to sample sequentially from different points for different periods of time.